Typically, electric brake systems include a mechanism for converting the rotary motion of an electric motor to a linear motion of a friction pad to press the friction pad against the brake disk, thereby generating a braking force. In order to control the braking force to a desired magnitude, a load sensor unit is mounted in many of such electric brake systems at a portion where a reaction force of the load applied to the friction pad is received. For improved responsiveness of braking, it is preferable to use a load sensor unit which can detect a load applied when its moving part or parts move a shortest possible distance.
A load sensor unit which can detect loads with a minimum displacement of its moving parts is disclosed e.g. in the below-identified Patent document 1. This load sensor unit includes an opposed pair of annular presser plates, piezoelectric crystal elements disposed between the presser plates, an insulating plate electrically insulating the piezoelectric crystal elements from one of the opposed pair of presser plates, and a lead wire through which voltage generated by the piezoelectric crystal elements are taken out.
When an axial load is applied to this load sensor unit, the piezoelectric crystal elements generate a voltage corresponding to the load applied. It is thus possible to detect the load applied by measuring the voltage generated. Since the presser plates are moved very little relative to each other due to deformation of the piezoelectric crystal elements, this sensor unit will never deteriorate responsiveness of braking if this sensor unit is mounted in an electric brake system.
But since this load sensor is designed such that loads act on the piezoelectric crystal elements too, if an impulsive load or a load oblique to the axial direction is applied to the piezoelectric crystal elements. one or more of the piezoelectric crystal elements may crack or chip. Also, since loads act on the insulating plate electrically insulating the piezoelectric elements and one of the presser plates too, high durability is required for the insulating plate. It is therefore difficult to use an inexpensive insulating material such as resin for the insulating plate because such an inexpensive insulating material is insufficient in durability.
A load sensor unit which can detect loads with a minimum displacement of its moving parts is also disclosed in the below-identified Patent document 2. The load sensor unit disclosed in Patent document 2 includes a cylindrical member made of a metal, and a strain gauge stuck on the radially outer surface of the cylindrical member. When an axial load is applied to this load sensor unit, strain is generated in the cylindrical metal member corresponding to the axial load applied. By measuring the magnitude of the strain generated, it is possible to detect the magnitude of the load applied.
This load sensor unit has a problem in that since the strain gauge measures not the degree of deformation itself of the cylindrical metal member but local strain of the cylindrical member, a change in temperature and the temperature distribution, of the cylindrical member tend to influence the detection result, and thus could result in errors in detection.